\documentclass{beamer}
\setbeamertemplate{navigation symbols}{} % Has anyone ever used these?
\begin{document}
\begin{frame}
\begin{tabular}{|l|r||l|r|}
\hline
\multicolumn{2}{|c||}{\onslide<2->{\textsc{Actif}}} & \multicolumn{2}{c|}{\onslide<2->{ \textsc{Passif}}}\\ 
\hline
\hline
\onslide<3->{Banque} & \onslide<3->{18550} & \onslide<4->{Capital} & \onslide<4->{18550}\\
\hline
\onslide<5->{\textbf{Total}} & \onslide<5->{18550} & \onslide<5->{\textbf{Total}} & \onslide<5->{18550}\\
\hline
\end{tabular}
\end{frame}
\end{document}
